When do you need a vitamin or a supplement? Some researchers say that a daily multivitamin may be a good idea for most adults, according to the Harvard School of Public Health.

That's not to say you should abandon good nutrition in your daily diet. If you eat a varied diet with lots of fruits and vegetables, you're likely to get all the vitamins and minerals that you need.

If you take a dietary supplement that focuses just on one or two nutrients, you may end up with too much of those nutrients, interfering with the absorption of other nutrients.

What are vitamins and minerals?

Vitamins are complex organic micronutrients, which is a fancy way of saying that they work at the microscopic level to help your body carry on complex chemical processes. They have no calories themselves, and with a few exceptions, they're not manufactured inside your body. Some vitamins help in biochemical transformations, such as absorbing oxygen in the lungs. Others act as "antioxidants," protecting tissues from disease-causing damage and premature aging.

Minerals are naturally occurring inorganic elements your body needs to function. Minerals allow the body to build some structures and help trigger some bodily reactions. Iron, for example, is a key to the production of hemoglobin in the red blood cells. Calcium is used to build bone and tissues. Zinc is essential for nerve transmission. Compared with vitamins, minerals have a very simple molecular structure. And unlike some vitamins, cooking does not destroy their essential nutrients.

Vitamin details

Here's a look at some of the important vitamins:

Vitamin A. This vitamin helps you see in the dark. It helps with the production of white blood cells, is important in bone health and regulates cell growth, say Harvard researchers. The current recommended intake of vitamin A is 3,000 international units (IU) for men and 2,310 IU for women. Fortified breakfast cereals, juices, and dairy products, as well as many fruits and vegetables contain vitamin A.

Vitamin C. Vitamin C helps control infections, as well as plays a role in bone, teeth, gums, and blood vessel health. The current recommended daily intake is 90 mg for men and 75 mg for women. Vitamin C is found in citrus fruits and juices, berries, bell peppers, tomatoes, broccoli, potatoes, and spinach.

Vitamin D. This vitamin is important for bone health, but many Americans don't get enough of this vitamin. Spending some time in the sun each day helps your body manufacture vitamin D. The requirement for this vitamin is 5 mcg up to age 50, 10 mcg from age 51 to age 70, and 15 mcg after age 70. Good sources of vitamin D are fortified dairy products and breakfast cereals, and salmon, tuna, and other fatty fish.

People in need

Some people can benefit from taking a supplement in addition to a healthy diet, however. According to the American Dietetic Association, a vitamin supplement may be helpful if you fit any of the following profiles:

You're a woman of childbearing age and don't eat fruits and vegetables.

You are pregnant.

If you believe you should take vitamin supplements, talk with your doctor or dietitian to make sure you are taking the correct amount and that the supplements are not causing an interaction with medications you take or conditions you have.